There are a number of vehicles, for example trucks, which have systems which consume, and are driven by, compressed air. In order for these systems to function, access to compressed air is necessary. Access to compressed air is usually achieved by a compressor which compresses air, which is then stored in pressure tanks where it is ready to be used by the compressed air users of the vehicle. The compressor is usually driven by the engine of the vehicle. Such a system needs to be fitted with a compressor, which increases the weight and fuel consumption of the vehicle. In order to make a vehicle financially more attractive, reducing the number of necessary components of the vehicle is of interest.
In a piston engine with a plurality of cylinders, in certain operational conditions one or more of the cylinders can be switched from normal combustion in order to temporarily be used for other purposes, such as for example an air compressor to fill compressed air tanks in a vehicle, which would replace a separate compressor. The compressor function is achieved by a cylinder room which can be connected to the compressed air tanks. This connection is closed during normal operation, and is opened when the cylinder is to be used as a compressor. When one or more cylinders are used as compressors, fuel supply to their corresponding cylinder space is cut off. When such a system is used, the pressure curve in the cylinder will have substantially different characteristics as compared to when the cylinder is used for conventional operation. During conventional operation, each cylinder has a compression stroke and an expansion stroke. During the expansion stroke, power is supplied to the system, and during the compression stroke the piston supplies power to the enclosed gas. If one or more cylinders are used to compress air, no normal expansion stroke will take place. This radically changes the pressure curve in the cylinder, and thus the torque which is transferred to the crankshaft of the engine. Due to the above mentioned changes of the pressure curve of the cylinder, the engine is not balanced in the same way as if all the cylinders were used for conventional operation. This causes the generation of vibrations with substantially different frequency components. A corresponding phenomena will occur when one or more cylinders are not used for their main purpose for other reasons.